Fig. 7

Kernel density curves of economic welfare efficiency. This kernel density plot depicts the dynamic evolution of absolute differences in economic welfare efficiency (Stage 2 of EWP) among CCUA cities. The horizontal axis represents efficiency values, and the vertical axis represents kernel density. The figure shows that the main peak of curves shifted left then right, consistent with the overall trend of economic welfare efficiency. Curve width expanded annually after 2016, indicating a slight expansion of absolute differences among cities. Notably, all years showed double peaks, reflecting persistent polarization: high-efficiency aggregation in northeastern CCUA and low-efficiency aggregation in the Chengdu metropolitan area.